All bag filling machines require the handling of the bag from a tight or closed condition to a puckered or open condition to allow a normally fluent product, e.g. cat litter, to be loaded into the bag, the bag then being reclosed to allow for final sealing. This process is accomplished by the use of a puckering system comprising a plurality of pairs of bag grippers. The grippers of each pair holding opposite top corners of the bag in spring loaded jaws. In a starting position the grippers are spaced well apart and hold the front and back panels taut and planar, resting flatly against each other with the bag flat and closed. When the two grippers move together, the bag puckers and the front and back panels can move apart, typically aided by front and back suction grippers that engage the respective panels and pull them apart transversely to the plane of the closed bag. The pairs of grippers normally follow a closed path through the filling machine with the closed bags lying in planes parallel to the transport direction.
The normal method for bag puckering with such bag grippers requires a considerable spacing between the trailing gripper of the leading pair and the leading gripper of the next trailing pair so as to not affect the next bag gripper pair when actuating the preceding pair. Usually the bag fed into the bag gripper puckering system has different sizes, more particularly in its transverse direction, i.e. at its top side which has to be opened for filling and to be closed after filling. Because of the different size of the bags, the top of the bag is not opened accurately, i.e. it is opened not enough, so that a part of the product to be filled into the bag is dumped next to the bag. Further, because of the different size, the closing of the top side will not be executed in an accurate manner, so that no accurate sealing is available some times.